Desorption ion sources, especially ion sources for ionization of samples by matrix-assisted laser desorption (MALDI), are used to ionize large molecules such as biomolecules or synthetic polymers. For special investigations, e.g., for mass spectrometric imaging of thin tissue sections, the ion sources must provide high sample throughput.
In desorption ion sources, a high-pressure desorption cloud is generated by bombardment of a sample with matrix and analyte material on a sample support plate with a short UV laser light pulse, usually a few nanoseconds in length. Analyte ions are formed in the plasma of the cloud by interaction with reactive ion species, which are generated from the desorbed matrix material in the desorption plasma as a consequence of the high laser energy input. After a largely uninfluenced expansion phase of some hundred nanoseconds, the ions are extracted from the desorption cloud by suddenly applying an accelerating field. The cloud, however, is not only a gaseous plasma; it sometimes also contains some splashes of solid or liquid matrix material from the quasi-explosion of the matrix material. As the cloud continues to expand, a portion of the vaporized and the splashed material is deposited on the first accelerating electrode, directly opposite the sample support, because of its close proximity of only a few millimeters to the MALDI sample support plate. But it is also possible that electrodes further downstream, a ground electrode, for example, may be affected.
When the desorption ion source is in operation, after several million laser shots, for example, a deposit of organic material builds up on the electrodes, sometimes visible by the naked eye. Such deposits in mass spectrometers are described in the literature by Girard et al. (Journal of Chromatography Science, 2010 October, 48 (9), 778-779) and Kenneth L. Busch (“Ion Burn and the Dirt of Mass Spectrometry”, online publication, Sep. 1, 2010). With increasing thickness of the insulating organic deposit a higher and higher surface charge is generated when the ion source is in operation, and thus generates an increasing electrical interference field which is superimposed onto the desired electric field between the electrodes and the MALDI sample support plate, and thus interferes with the acceleration process. A noticeable effect of such a deposit is a decrease in the ion throughput in the mass analyzer connected to the desorption ion source. The reduced ion throughput in turn requires the additional acquisition and summation of spectra in order to maintain a specific quality level for the mass spectra. The reduction in the ion throughput limits the number of analyses which are possible per sample and reduces the detection limit of the mass spectrometer, which interferes with or prevents semi-quantitative comparative measurements. Another noticeable effect is a reduction of the mass resolution of the mass spectra. The paper by Girard et al. identified above describes a method where reversing of the polarity of the ion source, which changes the polarity of the ions to be analyzed, can neutralize the charging effect. Since ions of both polarities are produced in a MALDI method, the polarity of the accelerating field would therefore have to be reversed for analogous application of the method according to Girard et al. This method, however, only addresses the symptoms of the loss of throughput in the ion source and promises only a short-term effect. In high-throughput operation with 2,000 laser shots per second, the deposits are charged up again in only a few seconds.
The acquisition of a mass spectrometric image of a rather small area of one centimeter times two centimeters of an thin tissue section with 25 micrometer resolution requires 400×800=320,000 sum spectra. If each sum spectrum is added up from 200 individual mass spectra, a total of 64 million individual spectra have to be acquired, with 64 million laser shots and 64 million desorption plasma clouds. There are laser beam profiles developed for modern MALDI ion sources that no longer produce splashes of matrix material, but even with these new ion sources, a few milligrams of matrix material are vaporized and partially deposited on acceleration electrodes during spectra acquisition. With modern MALDI ion sources operating with 2 kHz laser shot repetition rate, the total acquisition time amounts to about only ten hours; but may require easily several cleaning periods to maintain the quality of the spectra.
Irrespective of the short-term solution mentioned above, there is therefore regularly a need to remove the deposit and thus restore the performance level of the mass spectrometer. If the cleaning is not able to restore the proper state of the ion source, the ion source may even have to be replaced with a new, clean one.
A relatively reliable but old-fashioned method for removing the deposit, which is still used in practice, is to clean the electrodes manually after venting and opening the ion source. The cleaning is usually carried out with solvents such as ethanol or acetone, but the electrodes can also be abraded if the contaminations are stubborn, and this can be carried out with or without the removal of the accelerating electrode from the ion source. If the mass spectrometer is vented during the disassembly, it often takes some hours before the required operating vacuum is restored after the ion source has been cleaned and re-inserted. In addition, the time needed to readjust, in the worst case even to completely recalibrate, the mass spectrometer with the cleaned or exchanged ion source may have to be added to this.
In practice it is found that, after a time, the ion and electron bombardment causes the deposits on the electrodes of the ion source to harden; this is probably due to a polymerization of the organic substance. This means that cleaning the electrodes becomes more difficult the longer the operating intervals between the cleanings are. An objective of cleaning as often as possible to prevent the hardening effect is problematic. Economic considerations mean that the ratio of maintenance time to operating time should be as low as possible. The problem for the operator is to detect the contamination easily and with certainty, preferably before it hardens and therefore before becoming too difficult to remove.
Several proposals have already been made to integrate cleaning devices into the ion source so that it is not necessary to break the vacuum in order to clean the electrodes. U.S. Pat. No. 7,989,762 B2 to Holle et al, assigned to the assignee of the present invention, describes an automatic vaporizing of deposits by flash heating the central part of the acceleration electrode. Up to now, however, it depends on the judgment of the operator as to when cleaning is necessary. Because these cleaning methods suffer from the problem that they are hardly effective once the contamination has reached a certain degree of hardness, it is necessary to detect contamination early enough, before it attains a state which is no longer cleanable.
There is a need for the operator of a mass spectrometer with a MALDI ion source to be provided with a means or a procedure of quantitatively detecting contamination in the MALDI ion source, in order to determine when cleaning is necessary.